behavioral ecology of tropical birds - capitulo 5

7/28/2019 Behavioral Ecology of Tropical Birds - Capitulo 5

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5 T e r r i t o r i a l i t yLatitudinal differences in territorial behavior in relation to breedingseasons and mating system have had a great influence on latitudinal dif-ferences in life history strategies. In the temperate zone, territoryestablishment by the male, followed by mate attraction is the commonmodel of territory defense (Freed 1987). Temp ora ry breeding territo-ries, coupled with high over-winter mortality, mean that males andfemales have many unoccupied areas for establishing territories whenthey return in spring. For those that do retu rn about 50% of theirneighbors will be altogether new to them. Males and females have verydivergent interests due to the prevalence of extra-pair matings(Chapt er 4). In many species one or both sexes sneak off their terri toryto visit neighbors for extra-pair matings (Ne udo rf e t a l . 1997, Stutch-bury 1998b). Females seek out extra-pair matings, whereas males try toprevent their mates from doing this while at the same time seekingEPCs themselves. Males must defend their territories from frequentand sexually-motivated intrusions, even after territory boundaries arewell established. Territorial aggression is highest early in the nestingseason when territories are being established and EPC competition isat its peak (Arcese 1987).

In the tropics year-long territory defense is common and adultsurvival high, so breeding vacancies may be scarce. Consequently,birds face very different constraints in choosing mates and territories.It is commo n that either gender can maintain a territory, if their matedies or deserts them, and attract a new mate (e.g. Morton e t a l . 2000).Males and females each have similar interests in defending their terri-tory against same-sexed neighbors, who are after real estate rather tha nsex. Actual territorial intrusions are relatively infrequent, but borderdisputes and territory defense by singing can persist throughout theyear. Little is known about how territorial aggression varies over theseason, for either gender. Tropical territorial systems are not wellstudied, and are much more diverse than the 'simple' year-rounddefense described above. The costs and benefits of territory defensedepend on the type of territory and what is at stake. The general


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62 B E H A V I O R A L E C O L O G Y O F T R O P I C A L B I RD St e m p e r a t e z o n e m o d e l , w h i c h i s c o n s t r a i n e d b y c l i m a t e a n d s o d r i v e nb y ex t r a -p a i r m a t i n g b eh av i o r , ap p l ie s t o o n l y a m i n o r i t y o f t h e t r o p i ca lspecies .

5.1 T e r r i t o r y s y s t e m sO v e r 9 0 % o f N o r t h A m e r i c a n p a s s e r i n e s h a v e a s i m i la r te r r it o ri a ls y s t e m , t h e y d e f e n d b r e e d i n g t e r r i t o r i e s f o r o n l y a f e w m o n t h s e a c hs u m m er (T ab l e 5 .1 ) . I n t h e t r o p i c s , a s u s u a l , d i v e r s it y i s t h e n am e o f t h eg am e . O n l y 1 3 % o f p a s s e r i n e s d e f en d t e r r i t o r ie s o n l y fo r t h e b r eed i n gs e a s o n . I n s t e a d , t h e p r e d o m i n a n t t e r r i t o r i a l s y s t e m i s y e a r - r o u n dd e fen s e o f f e ed i n g a n d n e s t i n g t e r r i t o r i e s (T ab l e 5 .1 ) . T h i s t e r r i t o r i a ls y s t e m o c c u r s e v e n i n m o r e s e a s o n a l h a b i t a t s s u c h a s m a n g r o v e s(L e feb v re e t a l . 1 9 9 2 ). T h e t y p e s o f r e s o u r c e s d e f e n d e d , a n d w h e n t h e ya re d e f en d ed , is h i g h l y v a r i ab l e i n t h e t r o p i c s o w i n g to y ea r - ro u n d fo o dava i l ab i li ty an d th e d efens ib i l ity , o r l ack ther eof , o f d i f fe ren t food types .G en e ra l l y , i n s ec t iv o re s d e f en d y ea r - ro u n d a l l - p u rp o s e t e r r i t o r i e s , w h i l ef r u g i v o r e s d o n o t ( M o r t o n 1 9 7 3 , B u s k i r k 1 9 7 6 ) . Y e a r - r o u n d t e r r i t o r i -a l it y an d p a i r b o n d i n g is ty p i ca l o f t r o p i ca l i n s ec t i v o ro u s b i rd s , an do c c u r s i n 6 3 % o f p a s s e r i n e s in P a n a m a ( T a bl e. 5 . 1 ) a n d 4 0 % i nt r o p ic a l S o u t h A f r ic a ( R o w a n 1 9 6 6 ). A r t h r o p o d r e s o u r c e s a r e d e f e ns i-b l e b ecau s e t h ey a r e mo re o r l e s s ev en l y d i s t r i b u t ed s p a t i a l l y an dt emp o ra l l y .

T a b l e 5 . 1Ter r i to r ia l sys tems o f P anam an ian passerines com pared to Nor th Am er i can

passer ine b i rds.

Type o f Ter r i to ry a N u m b e r o f s pe cie s N u m b e r o f g e n e r a bP anama NA P anama NABreed ing 42 224 28 89Yea r- long 142 15 84 13Arm y an t i n f l uenc ed 11 0 9 0M ixed species f loc k 65 0 40 0Fru i t i n f l uence d 43 0 20 0Lek 28 0 19 0T o t a l 3 3 1 2 3 9 2 0 0 1 0 2a: See te xtb: Species n some genera fit mo re than one territory type (e.g., Elaenia, Vireo,Basileuterus, Sporop hila).


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TERRITORIALITY 63

Bu t t h e r e a r e man y v a r i a t i o n s w i t h i n t h i s b a s i c p a t t e rn . I n ad d i t i o nt o y e a r - l o n g t e r r i t o r i a l a n d p e r m a n e n t p a i r b o n d s y s t e m s , t h e t r o p i c soffer:

1. m an y t y p es o f l ek an d co o p e ra t i v e b r eed i n g ex amp l e s ,2 . l o o se l y d e f e n d e d t e r r it o r i e s w h e re p a i r m em b e r s l e av e w e l l-

d e f e n d e d n e s t i n g te r r i t o r i e s , m o re o r le s s i n d ep en d e n t l y , t o v is itf r u i t s o u rce s ,

3 . s p ec ie s m o r e o r le ss d e p e n d e n t u p o n t h e p e r e g r i n a t i o n s o f a r m yan t s , an d

4 . t h e p ro s an d co n s o f m em b e r s h i p i n mi x ed fl o ck s o f s ev era l d i f-fe ren t so r t s .

I n m i x ed f l o ck s o f t h e fo r e s t i n t e r i o r i n s ec t iv o ro u s t y p e , m an y s p ec i e sa r e r ep re s en t ed b y o n l y a p a i r o r s ma l l f ami l y p a r t y . T an ag e r / f l y ca t ch e rf lo ck s o f t h e fo r e s t c an o p y a r e mo re f ru g i v o ro u s b u t m i g h t s t il l b e r ep -r e s en t ed b y s ma l l n u m b e r s o f e ach s p eci e s. I n co n t r a s t , f r u i t - e a t i n gb i rd s o f m o re o p e n co u n t ry , o r l l an o s , ch a rac t e r i s ti c a l l y co n s i s t o f l a rg eg r o u p s o f c o n s p e c i f ic s ( M o y n i h a n 1 9 6 2, B u s k i r k 19 7 6 , M o r t o n 1 9 7 9 a ,M u n n a n d T e r b o r g h 1 9 7 9, P o w el l 1 9 7 9 ). T h e s e a r e d i s c u ss e d f u r t h e ri n C h a p t e r 7 .

T h e am az i n g d i v e r si ty o f t e r r i t o ry t y p es i n t r o p ica l f o r e s t s is b e s ts een b y co m p ar i n g s p ec ie s w i t h i n t h e s am e fo r e st (F i g u re 5 .1 ) . Sp ec i est h a t liv e in ca n o p y f l o cks , li ke t h e W h i t e - s h o u l d e r ed T a n ag e r , T a c h y -p h o n u s l u c t u o s u s , h av e t e r r i t o r ie s ab o u t f o u r t i mes t h e s iz e o f s p ec ie st h a t l iv e i n u n d e r s t o ry f lo ck s, li ke t h e W h i t e - f l an k ed A n t w ren , M y r -motheru la ax i l la r i s (F i g u re 5 .1 A , B) . O b l i g a t e an t - fo l l o w i n g b i rd s l i k et h e S o o t y A n t b i r d , M y r m e c i z a f o r t i s , h av e h u g e t e r r i t o r i e s (F i g u re5 .1 D ) c o m p a r e d w i t h o t h e r i n se c t iv o r o u s b i r ds t h a t d e f e n d y e a r - r o u n dt e r r i to r i e s a s p a ir s o r f ami l y g ro u p s (F i g u re 5 .1 C) . L ek k i n g s p ec ie s a r ec o m m o n , a n d e i t h e r f o r m t r a d i t i o n a l l e k s w h e r e m a n y m a l e s d i s p l a yv e r y c l o s e t o g e t h e r ( e . g . m a n y h u m m i n g b i r d s a n d m a n a k i n s ) , o r t h e yco ur t as ind iv idu a l s on so l i t a ry d i sp lay perc hes (F igu re 5 .1 E) as in theD w a r f T y r a n t - M a n a k i n , T y r a n n e u te s s t o l z m a n n i .

Te rr i to r y s i zes fo r t rop ica l b i rds a re typ ica l ly l a rger than w ha t we seef o r c o m p a r a b l e b i r d s i n t h e t e m p e r a t e z o n e ( T e r b o r g h et al. 1 9 9 0 ) . Fo ri n s t an ce , f o r e s t f ly ca t ch e r s , w ren s , r o b i n s , t an ag e r s an d v i reo s h av e t e r -r i to r i es tha t a re typ ica l ly 5 -1 5 ha in s ize . Th i s i s genera l ly 10 t ime sl a rg e r t h a n t h e a r e a d e f e n d e d d u r i n g t h e b r e e d i n g s e a s o n b y t h e i r t e m -p e r a t e z o n e c o u n t e r p a r t s ( T e r b o r g h et al . 1 9 9 0 ) . Ma l e s an d f ema l e se a c h d e f e n d t h e t e r r i t o r y a g a i n s t s a m e - s e x e d c h a l l e n g e r s , a n d d o n o t


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64 B E H A V I O R A L E C O L O G Y O F T R O P I C A L B I RD S

Figure 5.1Representat ive terr i tory types in a f loodplain forest in Amazonian Peru

(Terborgh et al . 1990 ). Shaded areas are territo ries .

usual ly coopera te in defending the te rr i tory (G reenb erg and Gr adw ohl1983 , F reed 1987, M or to n and Der r i ckson 1996) .

Turnover of adults on these terri tories is very low and this con-t r ibutes to remarkably s tab le te rr i tory boundaries (Greenberg andGradwohl 1997) . Lefebvre et al. (1992) suggested that terri tory stabi l-i ty may be low in seasonally versus permanently terri torial t ropicalspecies. However, we have found high site fidelity for several speciesthat seasonally defend ne st site terri tories. Clay-co lored R obins, Turdusgray i , defend terr i tor ies only during the dry season , and males andfemales bred on the same (33/36) or ad jacent te rr i tory (3 /36) in subse-quen t b reed ing seasons (S tu t chbury et al. 1998) . Lesser Elaenias ,El ae n i a ch i ri quensi s , in t ra tropica l m igrants tha t breed in the dry season ,a lso have h igh s ite tenaci ty wi th most re turn ing adul t s renest ing on thesame (6 /9) or an ad jacent (2 /9) te rr i tory (M orto n et al. unpub l ) . Thus ,


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TERRITORIALITY 65

t e r r i t o r i e s a re s t ab le f rom the pe r spec t i ve o f t he i nd iv idua l t ha t r eusesi t s t e r r i t o ry even i n spec i es wi th b reed ing t e r r i t o r i a l i t y . Ano the r con -t r i bu to r t o s t ab l e ne ig hbo rho ods is t ha t t rop i ca l spec i es wi t h yea r - l ongt e r r i t o r i e s d o n o t e x p a n d t e r r i t o r i a l b o u n d a r i e s , e v e n w h e n g i v e n t h eo p p o r t u n i t y t o d o s o e x p e r i m e n t a l l y ( M o r t o n e t a l . 2 0 0 0 ) . T h e r e a s o nfo r t h is is un s tud i ed , bu t pe rh aps b i rd s t ha t a re f am i l ia r w i th the i r t e r -r i to r i e s a re be t t e r ab l e t o avo i d p re da to r s (L im a 1998) .

F ac to r s t ha t de t e rm ine t e r r i t o ry qua l i t y fo r t rop i ca l b i rd s a re l i t t l es t u d ie d . F o o d a v a il ab i li ty d u r i n g t h e n o n b r e e d i n g p o r t i o n o f t h e y e a rm a y b e m o r e i m p o r t a n t i n d e t e r m i n i n g t e r r i t o r y q u a li t y a n d s iz e t h a nf o o d d u r i n g t h e b r e e d i n g p e r i o d . T h e r e a s o n i s t h a t b r e e d i n g s u c c es s isof ten low. A b i rd m igh t f ledge yo un g on ly once in it s li fe . We sp ecu la tet h a t t e r r i t o r y q u a l it y w i ll b e b a s e d u p o n n o n b r e e d i n g f a c to r s w h e nann ua l r e p rod uc t i ve succes s i s l es s t ha n 10%. T he n , t e r r i t o r i a l qua l i tytha t i nc reases i nd iv idua l su rv ivo r sh ip du r in g pe r i ods o f l ow food ab un -dan ce , o f t en t he d ry s eason , w i ll p reva il (M or ton e t a l . 2 0 0 0 ) . T e r r i t o r ys w i t c h i n g i n D u s k y A n t b i r d s , C e r c o m a c r a t y r a n n i n a , was r e l a t ed t oinc reas ing adu l t l i f e span and no t t o r ep roduc t i ve succes s pe r s e( M o r t o n e t a l . 2 0 0 0 ) .

In t e r spec i f i c t e r r i t o r i a l i t y som et im es occu r s where c lo se ly r e l a t edspec i es ( congener s ) com pe t e fo r yea r - l ong t e r r i t o r i e s i n t he m os tp r o d u c ti v e h a b it a ts . R o b i n s o n a n d T e r b o r g h ( 1 9 95 ) d o c u m e n t e din t e r spec i f i c t e r r i t o r i a l i t y u s ing r ec ip roca l he t e rospec i f i c p l aybacks i n10 o f 12 spec i es o f non -osc in e pas se r i nes i n P e ru . M os t o f t hese ha dn o n - o v e r l a p p i n g t e r ri to r i e s. S o m e o f t h e s a m e s p e c i es d o n o t h a v ein t e r spec i f i c t e r r it o r i a l i t y e l s ewhere (S tou f fe r 1997) .

M a n y i n s e c ti v o ro u s s p e c i es t h a t a r e p e r m a n e n t l y t e r r i to r i a l fe e d i nm ixed -spec i es f l ocks (P owel l 1985) . I t i s t he fo l i age -g l ean i ng and ba rk -g l ean ing b i rd s t ha t c l o se l y s c ru t i n i ze subs t r a t e s t ha t a re m os t t i ed t om ixe d -spec i es f l ocks, because p re da to r v ig i l ance is d if f icu l t t o m a i n t a inwi th t h is t ype o f fo rag i ng (P owel l 1985 ,Th io l l ay 1999) . Mixed -spe c i esf locks a llow b i rds to feed ef f ic ien t ly whi le t ak in g adv an tage of the v ig i -lance o f the f lock (Wi l li s 197 2) . G ene ra l ly a f lock con ta ins only a s ing lefam i ly g roup o f a pa r t i c u l a r spec ies , due t o s t rong t e r r i t o ri a l it y . S om espec i es de fen d pe rm an en t t e r r i t o r i e s sm a l l e r i n si ze t ha n t he f l ock , andthe l oca l t e r r i t o ry own ers j o in a nd l eave t he f l ock as i t m ov es ac ros s t e r -r i t o r y b o u n d a r i e s . T h i s m e a n s t h a t a g iv e n te r r i to r i a l p a ir m u s t s p e n dm uc h t i m e fo rag ing a lone on i ts t e r r it o ry , wh i l e the f l ock is e ls ewhere .O t h e r s p e c ie s ha v e te r r i to r i e s t h a t c o n f o r m t o t h e te r r i t o r y b o u n d a r y o ft h e m u l t i- s p e c i e s f lo c k ( M u n n a n d T e r b o r g h 1 9 7 9 , P o w e r 1 9 7 9 ,G r a d w o h l a n d G r e e n b e r g 1 9 8 0 ) . S o m e s p e c i e s ( e . g . a n t w r e n s )


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6 6 B E H A V I O R A L E C O L O G Y O F T R O P I C A L B IR D S

sometimes defend seemingly excessive territories, and have very lowpopulation density. This is because the flock's boundaries are deter-mined by the flock's larger species. Individuals of some smaller speciesbenefit greatly by defending the entire home range of the flock so thatthey can join it at any time.

Other permanently territorial insectivorous passerines are profes-sional ant-followers (Willis 1967, 1972). The sheer abun dance andspatial concentration of food at ant swarms makes it uneconomical toexclude conspecifics. Although multiple pairs of a given species may bepresent at an ant swarm, the male and female owners of the terri torywhere the ants are passing through are socially dominant over con-specifics. As the ants move into a neighboring territory, the owners ofthat territory become socially dominant at the swarm.

Many of the 42 species of Panamanian passerines exhibitingbreeding territoriality (Table 5.1) are frugivorous birds, which defendsmall nesting territories but feed off-territory on fruiting trees.However, 13% of the species in Panama are permanent ly paired anddefend year-long territories but leave them to visit fruit sources. InTable 5.1 this type of territoriality is called 'Fruit Influenced' toemphasize that fruit sources are ephemeral and not defended, eventhough the bird species have year-long territories and pair bonds (e.g.Yellow-bellied Elaenia, Elaenia f lavogaster) or year-long pair bondswithou t year-long territories (e.g. Blue-gray Tanager, Thraupis episco-pus ) . This contrasts with army ant influenced territories where pairsalso leave their territories to feed at swarms, because the food supply(fruit) is not defended by anyone even if it happen s to occur within apair's territory. For instance, Clay-colored Robin males will not attackother robins that enter their te rrito ry to feed on fruit, but only if thevisitors do not sing while they are there! Many other frugivores lek anddo not even defend nesting territories.

Nec tar is defensible because, unlike fruit, the food supply is rapidlyrenewed in flowers. In hummin gbi rds, long- term feeding territories areusually defended by males only (Wolf 1969, 1975). Intruders , male andfemale alike, are chased and attacked vigorously. In several speciesmales allow females to enter the t errito ry to feed, but only if the femalesallows the male to court her. In some species flowers are too scatteredto be defended, so males 'trap-l ine' by defending a series of high qualityflowers that they revisit on a predictable route.

Why aren't female hummingbirds also territorial? The spatialconcent ration of flowers allows males to defend nect ar not onlyfrom females, which are smaller, but also from other species of


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T E R R I T O R I A L I T Y 6 7

hummingbirds. This interspecific territoriality means that males canmonopolize high qual ity flower clusters. Females are forced to feed fromscattered flowers that are not economically defensible. The few hum-mingbirds where females defend long-term feeding territories featurebright female coloration (for defense) and are either large so femalescan dominate smaller hummingbird species or occur on islands wherethere are few interspecific competitors (Wolf 1969, 1975).

Territory defense of food by both sexes, and competition betweenthe sexes for limited food resources, can drive the evolution of sexualdifferences in resource use. The Purple-throated Carib ( E u l a m p i s j u g u -l a r i s ) , a hummingbird, is the sole pollinator of two H e l i c o n i a species(Temeles et a l . 2000). Remarkably, specialization by each sex of thehummingbird on different species of H e l i c o n i a has caused sexualdimorphism in bill size and shape.

5 . 2 T e r r i t o r y d e f e n s eD e f e n s e a n d e x t r a - p a i r c o p u l a t i o n sTerritory defense by temperate zone passerines is strongly influencedby the high frequency of extra-pair matings. In spring, males andfemales must acquire or reclaim a breeding territory when they firstarrive. Before the discovery of extra-pair mating systems, much of ter-ritorial behavior was thought to revolve around establishing andmainta ining the bo undar ies so neighbors did not take over part of theterritory. Once boundaries are set after a few weeks, strangers (e.g.floaters) were thought to be a bigger threat to the territory than neigh-bors (e.g. Wiley 1991). We have a different view of territoriality now.Males and females make frequent and covert forays onto neighboringterritories, not to take over the territory, but to seek copulations. InHoo ded Warblers, W i l s o n i a c i tr i n a , males and females leave their terri-tory about once every two hours to sneak onto a neighbor's territory(Neudorf et a l . 1998, Stutchbury 1998b). These are high stake intru-sions for males, because 2 0-5 0% ofyo ung are the result of EPC s.Th ereare winners and losers because some males father many extra-pairyoung in addition to those in their own nest, but other males father noyoung at all despite having a social mate and feeding the young in hernest (Stutchbury et a l . 1994, 1997). Neighbors are your worst enemy!

Breeding territories are set up nearly simultaneously within a popu-lation in the temperate zone spring, whereas, most tropical territorialsetting up is not synchronous. Less appreciated is the fact that many ofthe differences in territory defense between temperate zone and


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t r op i c a l pa s se r ine s a r e due to i n f lue nc e s o f e x t r a - pa i r be ha v io r . I n t e m-p e r a t e z o n e p a s s e r i n e s , i n t r u s i o n s a r e a t a c ti c o f e x t ra - p a i r m a t i n gs y s t e m s . M a l e t e r r i t o r y d e f e n s e i s t y p i c a l l y m u c h m o r e v i g o r o u s t h a nf e m a l e p e r h a p s b e c a u s e f e m a l e s f ac e a g r e a t e r t im e t r a d e o f f b e t w e e nr e p r o d u c t i o n a n d t e r r i to r i a l b e h a v i o r ( E l e k o n i c h 2 0 0 0 ) . T h e t e r r i t o ri a ls y s t e m o f t r o p i c a l p a s s e r i n e s i s b a s e d o n r e a l e s ta t e , b e c a u s e E P C s a r eso u n c o m m o n ( C h a p t er 4).

E P C s c a n b e a c c o m p l i s h e d i n a f e w m i n u t e s , s o m a l e v i g i la n c ea g a in s t i n t r u d e r s m u s t b e h i g h a n d p e rs i st e n t. E P C c o m p e t i t i o n m e a n st h a t i n t r u s i o n r a te s a r e h ig h , a n d o f t e n e s c a la t e i n to f i gh ts . I n H o o d e dW a r b l e rs , 2 0 % o f c o v e r t i n t r u s i o n s r e s u l t e d i n c h a s e s o r fi g ht s w i t h t h et e r r i t o r y o w n e r ( S t u t c h b u r y 1 9 9 8 b ) . A t y p i c a l t e r r i t o r y o w n e r f a c e sa b o u t o n e c o v e r t i n t r u s i o n p e r h o u r a n d is i n v o lv e d i n at l ea s t s e v er a lc h a s e s o r fi g ht s p e r d a y w i t h i n t r u d i n g m a l e s! E v e n w i t h o u t t h e a i d o fr a d i o te l e m e t r y , s u c h E P C c h a se s ar e c o n s p i c u o u s a n d c o m m o n l y s e e nin a w ide va r i e ty o f f o r e s t b i r d s .

I n t r o p i ca l p a s s e r in e s a c t u a l i n t r u s i o n s o n t o t e r r it o r i e s a r e m u c h l es sc o m m o n ( F r e e d 1 9 8 7 , G r e e n b e r g a n d G r a d w o h l 1 9 9 7 ). I n t e n s e f ig h ts ,w h e n o b s e r v e d , u s u a l l y i n v o l v e t a k e o v e r a t t e m p t s b y j u v e n i l e s o rf l o a t e r s , r a t h e r t h a n i n t e r a c t i o n s b e t w e e n n e i g h b o r s . B o r d e r d i s p u t e s ,w h e r e s i n g l e b i r d s o r p a i r s s i n g a t b o u n d a r i e s a r e c o m m o n b u t t h e s er a re l y e s ca l a te i n t o f ig h ts . P a ir s o f C h e c k e r - t h r o a t e d A n t w r e n s , M y r -m o t h e r u l a f u l v i v e n t r i s , f o r e x a m p l e , d e f e n d b o r d e r s o f t h e i r m i x e ds p e c i e s f l o c k t e r r i t o r y a g a i n s t c o n s p e c i f i c s w i t h l o u d a n d c o n t i n u o u sc h e h - c h e h - c h e h e t c . c a l l s a c c o m p a n i e d b y l a t e r a l b o d y m o v e m e n t s i nr h y t h m w i t h t h e c al ls . T h e s e c l a sh e s m a y l a st f or m a n y m i n u t e s .

M a l e s o n g o u t p u t is v e r y h i g h i n m o s t t e m p e r a t e p a s s e r i n e s . It f u n c -t i o n s i n e x t r a - p a i r m a t i n g c o m p e t i t i o n a s w e ll a s e s t a b li s h i n g t e r r i t o r yb o u n d a r i e s a n d a t t r a c t i n g a s o c i a l m a t e ( s e e C h a p t e r 6 ) . I n H o o d e dW a r b l e rs , m a l e s s p e n d a b o u t 5 0 % o f t h e i r t i m e s i n g i n g e v e n af t er t h e ya l r e a dy ha ve a soc i a l ma te ( W i le y e t a l . 1 9 9 4 , S t u t c h b u r y 1 9 9 8 b ) .F e m a l e s a s se s s th e q u a l i ty o f p o t e n t i a l e x t r a - p a i r p a r t n e r s b y a s s e s s in gt h e i r s i n g i n g o u t p u t a n d , i n s o m e , s o n g v a r i a b i l i t y ( K e m p e n a e r s e t a l .1 9 9 2 , H a s s e l q u i s t e t a l . 1 9 9 6 ) . M a t e c h o i c e c o n t i n u e s l o n g a f te r a m a l ea t t r a c t s a soc i a l ma te t o h i s t e r r i t o r y , so ma le s mus t ma in t a in a h ighs o n g o u t p u t . I n m o s t t r o p i c al p a s s e r i n e s s o n g o u t p u t is r e la t iv e l y lo w ,e v e n d u r i n g t h e s h o r t d a w n c h o r u s ( s e e C h a p t e r 6 ) . M a n y s p e c i e s a r eh e a r d o n l y b e t w e e n 0 6 1 5 a n d 0 7 3 0 o r f o r e v en s h o r t e r p er i od s . S o m es p e c ie s h a v e s p e c ia l d a w n s o n g s g i v e n b y m a l e s o n l y a t o r b e f o r e d a w n( S ta i c e r e t a l . 1996) .

E x t r a - p a i r m a t i n g a l s o h a s a n i m p a c t o n t e r r i t o r y s e t t l e m e n t


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pa t t e rns . F em ales p re fe r t o s e t t l e on t e r r i t o r i e s where ex t r a -pa i r m a l esa r e n e a r b y ( W a g n e r 1 9 9 3, 1 9 9 8 , W a g n e r e t a l . 1 9 9 6 ) . T h i s c a n r e s u lt ina c lu s t e r i ng o f t e r r i t o r i e s wi th f em ales av o id ing s e t t li ng on t e r r i t o r i e si so l a ted f rom po t en t i a l ex t r a -pa i r m a t e s .T h i s is c l ea rl y s een i n t he L eas tF l y c a t c h e r , E m p i d o n a x m i n i m u s , w h i c h h a s c o n s p i c u o u s c l u s t e r i n g o ft i ny t e r r i t o r i e s ( ' co lon i es ' ) t ha t sh i f t i n l oca t i on f rom yea r t o yea r( W a g n e r 1 9 9 8 , S . T a r o f , in p r e p . ). T h e h i g h c o s t o f o b t a i n i n g E P C sf rom i so l a t ed t e r r i t o r i e s m ay exp l a in why so m any fo res t pas se r i neso c c u r a t l o w f r e q u e n c y i n f o r e s t f r a g m e n t s t h a t a r e b i g e n o u g h t os u p p o r t a b r e e d i n g p a i r , b u t n e v e r t h e l e s s a r e u n o c c u p i e d ( M o r t o n1992 , Nor r i s and S tu t ch bury 2001) . We do no t expec t to s ee t h is EP C -based t e r r i t o ry s e t t l em en t p a t t e rn i n t rop i ca l passe r ines .T e s t o s t e r o n e a n d t e r r i t o r i a l i t yT e s t o s t e r o n e i s t h o u g h t t o b e t h e k e y p r o x i m a t e m e c h a n i s m d r i v i n gt e r r i t o r y d e f e ns e i n b i rd s . N u m e r o u s s t u d ie s o f t e m p e r a t e z o n e b i r d sh a v e s h o w n t h a t t e s t o s t e r o n e l e v el is h i g h d u r i n g t h e b r e e d i n g s e a s o n ,e s p e c i a l l y d u r i n g t e r r i t o r y e s t a b l i s h m e n t a n d c o u r t s h i p i n s p r i n g(Wingf i e l d e t a l . 1990) . C i rcu l a t i ng l evel s o f t e s t o s t e ron e co r re l a t e wi ththe i nd iv idua l ' s cu r re n t s t a t e o f agg res s ion on ly w he n soc i a l r e l a t ionsa re uns t ab l e , such as when t e r r i t o r i e s a re be ing es t ab l i shed , when t hem a t e i s s e x u al ly re c e p t iv e a n d w h e n m a l e s m a t e g u a r d ( W i n g f ie l d e t a l .1999) . Tes to s t e rone l eve ls i nc rease d ram at i ca l l y i n i nd i v idua l s cha l -l e n g e d b y i n t r u d e r s . I n d i v i d u a l s w i t h t e s t o s t e r o n e i m p l a n t s i n c r e a s ethe i r t e r r i t o ry s ize (Ke t t e r son e t a l . 1 9 9 2 ) . O u t s i d e t h e b r e e d i n g s e a s o n ,w h e n g o n a d s a r e r e g r e s s e d , i n d i v i d u a l s d o n o t i n c r e a s e t e s t o s t e r o n el eve ls i n r e sponse t o cha l l enges (Wi ngf i e ld 1994) . W i ngf i e ld and H ah n(1994) show tha t t e r r i t o r i a l agg res s ion can b e ' ac t i va t ed ' in the ab senceo f t e s t o s te r o n e i n a s e d e n t a r y p o p u l a t i o n o f S o n g S p a r r o w s , M e l o s p i z am e l o d i a . How ever , t hey i n s i st t ha t pe r s i s t ence o f agg res s ion i n t he f aceo f a ' s im u la t ed t e r r i t o r i a l i n t ru s ion ' is s ti ll t e s t o s t e r on e-d ep en de n t(Wingf ie ld e t al. 1999) .

T rop i ca l b i rd s b reak a l l t hese ru l e s . Many t rop i ca l b i rd s have ve rylow t e s t o s t e ron e l eve ls a ll yea r desp i t e be i ng h igh ly agg ress ive a nd t e r -r i to r i al ( D i t t a m i a n d G w i n n e r 1 9 9 0 , L e v i n a n d W i n g f ie l d 1 9 9 2 ,Wingf i e ld e t a l . 1 9 9 2 , W i n g f i e l d a n d L e w i s 1 9 9 3 ) . I n P a n a m a , S p o t t e dAntb i rd t e s t o s t e rone l eve l s ( and gonad s i ze ) r em ain ve ry l ow yea r -r o u n d e v e n t h o u g h i n d i v i d u a l s d e f e n d p e r m a n e n t t e r r i t o r i e s ( F i g u r e5 .2 ; W ikelsk i e t a l . 1999a) . Th e s easona l pa t t e rn o f t e s t o s t e ron e i s i ns h a r p c o n t r a s t w i t h a t e m p e r a t e z o n e b i r d l ik e t h e R e d - w i n g e d B l a ck -b i rd , , 4 g e l i a u s p h o e n i c e u s ( F i g u r e 5 .2 ) o r W h i t e - c r o w n e d S p a r r o w ,


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70 BEHAVIORAL ECOLOGY OF TROPICAL B IRDS

Figure 5.2Plasma testosterone level (ng m1-1) of male Sp otted Antb irds in Panama over th eyear, including the breeding season from April-N ovem ber (from W ikelski et a l .1999a), and for Red-winged Blackbirds in North Am erica th at breed fromAp ril-Ju ne (Johnsen 1998). Drawing s from Medsger (1931) and Wetm ore (1972).

Z o n o t r i c h i a l e u c o p h r y s ( H a u et al. 2 0 0 0 ) . T e s t o s t e r o n e i n S p o t t e dA ntb i rds be c a m e e l e va t e d on ly af t e r p ro lo nge d c ha l le nge s (> 90 min o fp l a y b a c k e x p e r i m e n t s ) , w h e t h e r o r n o t g o n a d s w e r e e n la r g e d , b u t e v enthe n the m a x im um e ve r re c or de d w a s 1 .5 ng m1-1. Th i s is m uc h low e rtha n t e s tos t e rone l evel s t yp i ca l o f t e m pe ra t e z one b i rds (4 -6 ng ml-1 ;H a u et al. 2 0 0 0 ) . T h e W h i t e - b r o w e d S p a r r o w W e a v er , Plocepasserm a h a l i , i n Za m bia ha s a s imi l ar l ow le ve l o f t e s tos t e rone ye a r - r oun d ,but in th i s spec ies tes tos te rone leve ls remain low even a f te r s imula tedin t rus ions (Wingf i eld a nd Le w is 1992) a nd e xp e r ime n ta l l y i nd uc e dma le t a ke ove rs (W ingf ie ld et al. 1992) .

E i the r t rop i c a l b i rds do no t ne e d t e s tos t e rone fo r song a nd a ggre s -s ion, or they a re h ighly sens i tive to very low leve ls of tes to s te ro ne (H auet aL 2000) . Te s tos t e ron e im pla n t s d id i nc re a se song a nd a ggre ss ion inc a p t ive Sp o t t e d A ntb i rds , a nd phys io log ic a l b loc k ing o f t e s tos t e roner e s u l t e d i n l o w e r s o n g o u t p u t a n d a g g r e s s i o n ( H a u et al. 2 0 0 0 ) . T h i se xpe r ime n t show s tha t t e s tos t e rone c a n a f fe c t song a nd a ggre ss ion int rop i c a l b irds , t hou gh the T impla n t s i nc re a se d t e s tos t e rone l evel s t oa bo u t 6 ng m1-1 , m uc h h ighe r t h a n is obse rve d na tu ra ll y . Suc h h igh


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TERRITORIALITY 71

l ev e ls o f t e s t o s t e ro n e , s een ro u t i n e l y i n t emp e ra t e zo n e b i rd s , c a r ry ah i g h p r ic e i n t e r m s o f i m m u n o s u p p r e s s i o n a n d o t h e r t r a d e o ff s( r ev iew ed i n Fo l s t ad an d K a r t e r 1 9 9 2 , W i n g f i e l d e t a l . 1 9 9 9 ) . T ro p i ca lb i rd s can c l ea r l y d e f en d an d ma i n t a i n t e r r i t o r i e s w i t h o u t h i g h t e s t o s -t e ro n e l ev e l s , an d i n s o me s p ec i e s ma l e s can e l ev a t e t e s t o s t e ro n eo p p o r t u n i s t i c a l l y a f t e r p ro l o n g e d t e r r i t o r i a l ch a l l en g es (W i k e l s ki e t a l .1 9 9 9 a ) . S u ch ' s o c ia l i n s t ab il i ty ' o ccu r s a t lo w f r eq u e n cy i n tr o p i ca l t e r -r i t o r i a l s y s t ems , l a rg e l y b ecau s e ma l e s a r e n o t co mp e t i n g w i t h eacho t h e r f o r e x tr a - p a ir c o p u l a t i o n s ( se e C h a p t e r 4 ). W i t h o u t t h e s t r o n gs ex u a l s e l ec t io n f ro m ex t r a -b eh a v i o r t h e r e w o u l d b e n o b en e f i t t o ma i n -t a i n i n g e l ev a t ed t e s t o s t e ro n e l ev e l s . Fema l e s a l s o s i n g an d d e fen dt e r r i to r i e s , b u t n o t h i n g is k n o w n ab o u t t h e ro l e o f t e s t o s t e ro n e , o r o t h e rh o r m o n e s , i n f e m a l e s ( H a u e t a l . 2 0 0 0 ) .R e s o u rc e h o l d i n g p o t e n t i a lA n o t h e r t r o p i c a l / t e m p e r a t e d i f f e r e n c e c o n c e r n s t h e r e s o u r c e h o l d i n gp o t e n t ia l ( R H P ) o f t e r r i to r y o w n e r s . G a m e t h e o r y m o d e l s o f t e r r i to r yd e fen s e d i s t i n g u i s h RH P, t h e p h y s i ca l ab il it y t o d e f en d an d f ig h t , f r o mres o u rce v a l u e (RV ) w h i ch is t h e m o t i v a t i o n t o f ig h t o w i n g to t h e v a l u eo f t h e t e r ri t o r y to t h e c o n t e s t a n t ( M a y n a r d S m i t h a n d P a r k e r 1 9 7 6 ).T o g e t h e r , R H P a n d R V h e l p t o p r e d i c t a s y m m e t r i e s b e t w e e n c o n t e s -t an t s a n d t h e r e fo r e t h e o u t co m e o f co n t e s t s a s w el l a s t h e l i k e l ih o o d o fe s ca l a ti o n . T h es e mo d e l s o f t e r r i t o ry d e f en s e a r e u s u a l l y s t u d i ed ex p e r -i men t a l l y b y ex am i n i n g t h e ab i l it y o f r em o v e d o w n e r s t o r eg a i n t h e i rt e r ri to r i e s f r o m r e p l a c e m e n t s . T e m p e r a t e z o n e r e m o v a l s s h o w t h a t t h ep ro b ab i l i t y t h a t r ep l acemen t i n d i v i d u a l s w i l l d e f ea t a f o rmer r e s i d en ti n c r ea s e s w i t h r ep l acemen t t i me (K reb s 1 9 8 2 , Be l e t s k y 1 9 9 6 ) . A nab s en ce o f 4 8 h i s en o u g h t o t ip t h e b a l an ce i n f av o r o f t h e r ep l acem en t ,w h o i s u s u a l l y a f l o a te r r a t h e r t h a n a fo rme r t e r r i t o ry o w n e r . O n l y 1 6 %o f R e d - w i n g e d B l a c k b ir d m a l e s r e g a i n e d t h e i r t e r r it o r y a f t e r b e in gd e t a i n e d f o r 6 - 7 d , c o m p a r e d w i t h 9 1 % s u c c es s in tw o - d a y r e m o v a l s(Be l e ts k y an d O r i an s 1 9 8 7 , 1 9 8 9 ) .

Few ex p e r i me n t s l ik e t h is h av e b een p e r fo r m ed i n t r o p i ca l b i rd s. T h ev a l u e o f b r eed i n g t e r r i t o r ie s o f t em p e ra t e z o n e b i rd s m i g h t d i f fe rg r ea t ly f ro m t h e y ea r - l o n g t e r r i t o r i e s o f tr o p i ca l b i rd s. D u s k y A n t b i rd so f e it h e r g en d e r a lw ay s r eg a i n ed t h e i r t e r r i to r i e s , a f t e r t h ey w e rer e l ea s ed f ro m cap ti v it y , r eg a rd l e s s o f r ep l a cem en t t i me u p t o 1 0 d ay s( M o r t o n e t a l . 2 0 0 0 ) . T h e r e p l a c e m e n t s o f t h e r e m o v e d t e r r i to r yo w n e r s w e re o t h e r t e r r i t o ry o w n e r s (n o t f l o a t e r s ) , w h i ch s w i t ch edm a t e s an d o l d t e r ri t o r ie s f o r t h e n ew t e r r it o ry . T h e r ep l ac em en t s , t h e r e -f o re , h a d t h e o p t i o n o f r e t u r n i n g t o t h e t e r r i to r y t h e y e m i g r a t e d f r o m


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7 2 BEHAVIORAL ECOLOGY OF TROPICAL BIRDSa n d o u s t i n g t h e i r r e p l a c e m e n t s , i f a ny . R e m o v e d r e s id e n t s , i n c o n t r a s t ,d id n o t have a n a l t e rna t i ve bes ides r ega in i ng t he i r t e r r i to ry . T her e l e a s ed o w n e r s ' m o t i v a t io n t o f ig h t m u s t h a v e b e e n h i g h e r t h a n t h e i rr e p l a c e m e n t s ' m o t i v a t i o n o w i n g to t h is a s y m m e t r y .

5 . 3 T e r r i t o r y a c q u i s i t i o nI n t e m p e r a t e s p e c i e s , r e m o v a l e x p e r i m e n t s h a v e s h o w n t h a t y o u n gb i rds o f m an y spec i es op t to de l ay b ree d ing as non - t e r r i t o r i a l b i rd s ,e i t he r beca use no b re ed in g pos i t ions a re ava il ab le ( e .g . S tu t ch bu ry an dR o b e r t s o n 1 9 8 7 a , S t u t c h b u r y 1 9 9 1 ) o r i n a n a t t e m p t to g a in a h i g hq u a l i ty b r e e d i n g p o s i t i o n m u c h a s c o o p e r a t iv e b r e e d e r s d o ( Z a c k a n dS t u t c h b u r y 1 9 9 2 ) . T a c t ic s f o r g a in i n g te r r i to r i e s i n c l u d e w a n d e r i n gwide ly fo r vacanc i es (S t u t c hb ury an d R o be r t so n 1987a) , li v ing s ec re t ly( A r c es e 1 9 8 9 ) o r o p e n l y ( E c k m a n 1 9 8 8) o n t h e t e r r it o r ie s o f b r e e d e r s ,o r ev i c ti ng t e r r i t o ry ow ners ou t r i gh t (Arcese 1987) . F o r m os t t rop i ca ls p e c i e s r e m o v a l e x p e r i m e n t s h a v e r a r e l y b e e n p e r f o r m e d ( M o r t o n1 9 7 7 a , L e v i n 1 9 9 6 a , M o r t o n et a l . 2 0 0 0 ) a n d w e do n o t k n o w w h e t h e rf l oa te r s even ex is t, l e t a lone how you ng b i rd s ob t a in t he i r f i rs t te r r i t o ry .Ter r i t o ry acqu i s i t i on has been ca re fu l l y s t ud i ed i n coopera t i ve lyb r e e d i n g b i r d s, m a n y o f t h e m t r o p ic a l , w h e r e y o u n g o f t e n i n h e r i t t h e i rna t a l t e r r i t o ry o r u se t he i r na t a l t e r r i t o ry a s a r e fuge f rom whi ch t oc o m p e t e f o r b r e e d i n g p o s i t i o n s o n n e a r b y t e r r i t o r i e s ( Z a c k 1 9 9 0 ) .T h e r e a r e m a n y s i m i l a r i t i e s b e t w e e n c o o p e r a t i v e b r e e d e r s a n d o t h e rs p e ci e s in h o w y o u n g g o a b o u t g e t t in g a b r e e d i n g p o s i t io n ( Z a c k a n dS t u t c h b u r y 1 9 9 2) .

In s t ea d o f w and er in g wide ly ( e .g . ' f loa t i ng ' in t he t rue s ense ) n on -b r e e d e r s i n s o m e r e s i d e n t t e m p e r a t e s p e c i e s g a i n a c o m p e t i t i v ea d v a n t a g e f o r b r e e d i n g v a c a n c i e s b y a s s o c i a ti n g c l os e ly w i t h o c c u p i e db r e e d i n g t e r r it o r ie s , v ia th e s a m e k i n d o f si te d o m i n a n c e a d v a n t a g e t h a tt e r ri t o ry o w n e r s e n jo y ( B i rk h e a d a n d C l a r k s o n 1 9 8 5, E c k m a n 1 9 8 8,M at thy sen 1989) . Th i s t ac t i c was fi rs t desc r i be d i n a t rop i ca l b i rd , t heR u f o u s - c o l l a r e d S p a r r o w , Z o n o t r i c h i a c a p e n s i s , i n C o s t a R i c a ( S m i t h1 9 7 8 ) w h e r e a n ' u n d e r w o r l d ' o f n o n b r e e d e r s liv e f u rt iv e ly o n t h e t e rr i -t o r ie s o f b r e e d e r s w a i t in g f o r a v a c a n c y t o a r is e. N o n b r e e d e r s h a v ew e l l - d e f i n e d h o m e r a n g e s t h a t t h e y d e f e n d f r o m o t h e r n o n b r e e d e r s ,a n d w h e n a b r e e d e r d i s a p p e a r s t h e r e p l a c e m e n t i s a n o n b r e e d e r w h o s eh o m e r a n g e i n c l u d e d t h a t t e rr i to r y . T h i s k i n d o f n o n b r e e d e r t a c t ic f o rt e r r i t o r y a c q u i s i ti o n is li ke ly t o b e c o m m o n w h e r e y e a r - r o u n d t e r r i t o r ydefense and h i gh food ava i lab i li ty m ake i t pos s ib l e fo r young b i rd s t oq u e u e f o r a b r e e d i n g p o s i ti o n .


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I n t ro p i c a l H o u s e W r e n s , T r o g l o d y t e s a e d o n , f loa te rs o f bo th sexes a ret r an s i en t s an d h av e a t l e a s t t w o ro u t e s t o t e r r i t o ry acq u i s i t i o n (F reed1 9 8 6 , 1 9 8 7 ) . S o m e w a i t p a s s iv e l y fo r v acan c i e s t o o ccu r , b u t ad u l t s a r elong- l ived so vacanc ies a r i se ra re ly and a re f i l l ed very rap id ly . F loa te rsa ls o a t t em p t t o ev ic t t e r r i t o ry o w n e r s i n s t ead o f w a i t i n g fo r t h e m t o d i e ,an d k il l t h e i r n e s t li n g s i f t h e t ak eo v e r o ccu r s d u r i n g b r eed i n g . F l o a t e r ss o m e t i m e s f o r m p a i r b o n d s , a n d t h e n t a k e o v er te r r it o r ie s a s a t e a m .

I n m a n y m o n o g a m o u s t r o p ic a l b i r ds , j uv e n il es l iv e o n t h e i r p a r e n t ' st e r r i t o ry fo r s ev era l t o m an y mo n t h s ( e.g. Ro b i n s o n e t a l . 2 0 00 , M o r t o ne t a l . 2 0 0 0 ) , a n d t h is c o u l d g iv e t h e m a n e d g e i n c o m p e t i n g f o r n e a r b yv acan c i e s t h a t c an a r i s e an y t i me o f y ea r. I n Ch eck e r - t h ro a t edA n t w ren s , j u v en i l e s l i v e w i t h t h e i r p a r en t s f o r a s h o r t t i me (1 -2m o n t h s ) , b u t u s e t h i s a s a h o m e b a s e f r o m w h i c h t o c h a l l e n g e n e i g h -b o r i n g t e r r i t o r y h o l d e r s f o r o w n e r s h i p ( G r e e n b e r g a n d G r a d w o h l1 9 9 7 ) . S u ch ch a l l en g es i n vo l ve l o n g co n t e s t s o f d i s p lay s an d ch as e s ,b u t r a r e ly r e s u l t i n t ak eo v e r s ( u n l ik e H o u s e W ren s ) . Te r r i t o ry acq u i s i -t i o n i s co n s t r a i n ed b y t h e s p ec i a l iz ed ae r i a l d ead - l ea f f o r ag in g b eh av i o ro f t h is s p ec ie s . Fo o d r e s o u rce s a r e s ca r ce w i t h i n a t e r r i t o ry o w i n g to t h el i mi t ed av a i l abi li ty o f d ead l eav es, s o y o u n g can n o t liv e o n t h e i r p a r en t s 't e r r i t o ry fo r v e ry l on g . H o w ev e r , l iv i n g a l o n e i s r i sk y b ecau s e d ea d - l ea ffo r ag i n g mak es i t d i f f i cu l t t o b o t h s ea r ch fo r f o o d i n s i d e d ead l e av esan d b e v i g i l an t f o r p r ed a t o r s . Th i s s p ec ie s j o in s m i x ed - s p ec i e s f lo ck s tor ed u ce t h e r i sk o f p r ed a t i o n w h i l e f o r ag in g . M o s t j u v eni le s t h a t w e reb a n d ed s e t t led v e ry c l o se t o t h e i r n a t a l t e r r it o ry .

S p o t t e d A n t b i r d , H y l o p h y l a x n a e v i o i d e s , y o u n g a l s o l e av e t h e i rp a r e n t s ' t e r r i to r y a f t e r o n l y 6 - 8 w e e k s, p e r h a p s o w i n g to c o m p e t i t i o nw i t h ad u l t s a t an t s w a rms . I n t h i s s p eci e s, y o u n g b i rd s d o n o t s e t tl e n ea rt h e i r p a r en t s ' t e r r i to ry , an d i n d i v i d u a l s t h a t f il led v acan c i e s c a me f ro mo u t s i d e t h e s t u d y a r ea o n P i p e l i n e Ro ad , i n Pan ama ( J . N es b i t t , p e r s .c o m m . ) . M a n y n o n - t e r r i t o r i a l ' f l o a t e r s ' t h a t w e r e b a n d e d w i t h i n t h es t u d y s i t e ev en t u a l l y acq u i r ed b r eed i n g t e r r i t o r i e s t h e r e , s u g g es t i n g as y s t e m s i m i l a r t o t h e ' u n d e r w o r l d ' d e s c r i b e d f o r R u f o u s - c o l l a r e dS p a r r o w s ( S m i t h 1 9 7 8 ) .

I n D u s k y A n t b i r d s f lo a te rs a r e u n c o m m o n , p r o b a b ly b e c a u s e y o u n gb i rd s l i v e w i t h t h e i r p a r en t s u p u n t i l t h e n ex t b r eed i n g s ea s o n an dr e p r o d u c t i v e s u c c e ss is v e r y l ow ( M o r t o n a n d S t u t c h b u r y 2 0 0 0 ,M o r t o n e t a l . 2 0 0 0 ) . M a n y t e r r i t o r i e s r e m a i n e d u n o c c u p i e d a f t e r a no c c u p a n t w a s e x p e r im e n t a l l y r e m o v e d d u r i n g t h e n o n b r e e d i n g s e a s o n ,o r w h en t h e i r o ccu p a n t s d i s ap p ea red n a t u r a ll y . Th i s s u g g es t s th e r e i sn o s h o r t ag e o f t e r r it o r i e s. N ew t e r r i to r i e s w e re e s t ab l i s h ed b y p a i r s o fj u v en i l e s , n ev e r b y s i n g l e b i rd s , w h i l e ' w i d o w ed ' ad u l t s r ema i n ed o n


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t h e i r t e r r it o r i e s a n d a d v e r t i s e d fo r a n e w m a t e t o jo i n t h e m ( M o r t o n e ta l. 2 0 0 0 ) . I n D u s k y A n t b i r d s , t e r r i t o r y e s t a b l i s h m e n t i s l i k e l y c o n -s t r a i n e d b y p r e d a t i o n . P a i rs f o r ag e t o g e t h e r i n d e n s e h a b i t a t a n d d o n o tjo in mixe d - spe c i e s f l oc ks , so l i v ing a lone on a t e r r i t o r y ma y e xpose ab i r d t o a v e r y h i g h r i s k o f p r e d a t i o n b y a m b u s h i n g p r e d a t o r s l ik e v i n es n a k e s a n d b o a s . T h e q u a l i t y o f a t e r ri t o r y , t o a p o s s i b le n e w c o m e r , m a yb e g r e a t e r i f a n e x p e r i e n c e d r e s i d e n t i s p r e s e n t o n it . S u c h a r e s i d e n tm a y b e f a m i l i a r w i t h p r e d a t o r s a n d t h e i r lo c a t io n s o n t h e t e rr i to r y . W ep r e d i c t t h a t , i f o n e r e m o v e s b o t h r e s i d e n t s f r o m a t e r ri t o r y , th e q u a l i t yo f t h a t t e r r i t o r y w i ll b e r e d u c e d o w i n g t o t h e h i g h c o s t o f li v in g a l o n e ,a n d i t w ill r e m a i n u n o c c u p i e d . T h e s e t o ta l r e m o v a ls c a n b e c o m p a r e dw i t h p u b l i s h e d d a t a o n r e p l a c e m e n t r a t e s w h e r e o n l y s in g l e i n d i v i d u al sw e r e r e m o v e d ( M o r t o n et a l . 2 0 0 0 ) t o t e s t t h e ' e x p e r i e n c e d r e s i d e n tinc r e a se s t e r r i t o r y qua l i t y ' hypo the s i s . W e p r e s e n t t h i s hypo the s i s t os t i m u l a t e t h i n k i n g a b o u t t h e s e t r o p i c a l t e r r i t o ri a l s y s te m s . Y e a r - lo n gt e r ri t o r ia l i ty i s, a ft e r al l, t h e m o s t c o m m o n f o r m o f t e r ri t o r ia l i ty w o r l d -w i d e a n d w e k n o w a l m o s t n o t h i n g a b o u t s o u r c e s o f s e l e ct i o n a c t in gu p o n it.

H o w j u v en i le D u s k y A n t b i r d s j o in t o g e t h e r to s e t u p t e r r it o r i e s a n dh o w l o n g t h e y a r e t o l e r a t e d o n t h e i r p a r e n t s ' t e r r i t o r i e s a r e n o t w e l lk n o w n . T h e y a p p e a r e d t o f o r m p a i r b o n d s w i t h ju v en il es o n a d j a c e n tt e r r i t o r i e s a n d u s e s p a c e c o n t i g u o u s t o b o t h p a r e n t a l t e r r i t o r i e s . I no t h e r w o r d s , t h e i r t e r r i t o r y w a s b u d d e d o f f a t e r r i t o r y f r o m t o l e r a n tp a r e n ts . W h e n w e a t t e m p t e d t o c a p t u r e o n e s u c h p a i r f o r b a n d i n g , t h em o t h e r o f o n e o f t h e p a i r e d j u v en i le s l ef t h e r t e r r i t o r y a n d w a sc a p t u r e d ! P e r h a p s s h e w a s ' h e l p i n g o u t ' t h e d a u g h t e r .

O t h e r y e a r - r o u n d t e r r it o r i a l p a s s e r in e s w i t h ju v e n il e r e t e n t i o n d i ff e ri n s o m e d e ta il s . B u f f - b r e a s t e d W r e n , T h r y o t h o r u s l e u c o t i s , r e m o v a l sr e s u lt e d i n 1 0 0 % r e p l a c e m e n t e i t h e r b y b a n d e d y o u n g o r n e i g h b o r i n ga d u l t s , a n d s o m e t i m e s u n b a n d e d f l o a t er s , u s u a l ly w i t h i n 2 4 h ( S. G i lla n d B . J. M . S t u t c h b u r y , u n p u b l . ) . F l o a t er s a r e u n c o m m o n , b u t d oo c c u r . A l l a d u l t s a n d t h e i r y o u n g w e r e b a n d e d i n t h is p o p u l a t i o n , a n do c c a si o n al ly u n b a n d e d b i rd s w e r e o b s e r v e d m o v i n g t h r o u g h t e rr i to r i eso r s i n g i n g a l o n e f r o m a s m a l l a re a . I n W h i t e - b e l l i e d A n t b i r d s , M y r m e -c i z a l o n g i p e s , t h o u g h , e x p e r i m e n t a l r e m o v a l s o f t e n d i d n o t r e s u l t i nr e p l a c e m e n t s , sug ge s t ing f e w f loa t e rs e x i s t i n e i t he r s e x (B . F e d y a n d B .J . M . S t u t c h b u r y , u n p u b l . ) . T h e s e e x p e r i m e n t s h a v e r e v e a l e d a g r e a tv a r i et y a m o n g s p e c ie s t h a t o f te n o c c u p y t h e s a m e h a b i t a t s , i n t e r m s o ft h e f r e q u e n c y o f f l oa t e rs , h o w j u v en i le s g o a b o u t g e t t i n g t e rr i t o r ie s , a n dh o w f a r j u v en i le s g o f r o m h o m e t o g e t b r e e d i n g p o s i t i o n s. W h y a r eW h i t e -b e l l ie d A n t b i r d s s o d i ff e re n t f r o m D u s k y A n t b i r d s . . . w e d o n ' t


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know! This is so often the answer to questions we, and our students,pose about tropical birds.

5 .4 Ter r i tory sw i tch ingYear-round territories have remarkably stable boundaries from year toyear, even when owners are replaced (Greenberg and Gradwohl 1986,1997). But permanent territoriality does not mean stasis. Severalstudies have found that t errit ory switching occurs at a low rate in year-round residents (Willis 1974, Freed 1986, Greenberg and Gradwohl1986, 1997,Woodworth e t a l . 1999, Morton e t a l . 2000).

But switching is an important aspect of territoriality, because thehigh adult longevity means that 25-50% of adults switch territoriesonce during their lifetime (Greenberg and Gradwohl 1997, Morton e ta l . 2000). Pair bonds in Dusky Antbirds, apparently stable, are quicklybroke n when vacancies arise on nearby territories (Mor ton e t a l . 2000).We removed males or females from territories, then monitored whofilled those vacancies and how quickly this occurred.

Figure 5.3 illustrates how vacancies on some territories are hotlycomp eted for, while others are ignored. The male from te rrito ry A wasreplaced by an adjacent male in less than 12 h, which was in turn oustedby an unba nded male. We then removed the un ban ded male and thesame adjacent male moved back to reclaim the vacancy; he wasreplaced on his former territory by a yearling neighbor male within12 h. Wh en the original owner of A was released (after 168 h in captiv-ity) he reclaimed his terr itory.The adjacent male retu rne d to his formerterritory, as did the yearling male. In stark contrast, a removal on t erri-tory D resulted in no replacements after 70 h, and a naturaldisappearance of the male on territory E left that female unma ted for2.5 months!

Territory switching was equally common and rapid among males andfemales (Figure 5.4). Whe n the original owners were released theyalways won back their territory, and replacements went back to theirformer territory, in a domino fashion. On average an individual switchedterritories about once in its lifetime. However, some individualsremained for many years (up to 10) on the same territory, suggestingthese territories were preferred. Terri tory switching by breeding adults isprobably a common tactic for gaining higher quality territories in manytropical birds. Levin (1996a) also docume nted several cases of terr itoryswitching by males and females after doing removal experiments in theBay Wren, T h r y o t h o r u s n i g r i c a p i l l u s . In a congener, the Buff-breasted


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76 BEH AVIO RAL ECO LO G Y O F TRO PICAL B IRDS

Figure 5.3Outcome of male removal experiments (A-D) and one natural disappearance (E)in the Dusky An tbir d durin g the non bree ding season (Mo rton et al. 2000). Terri-tories (solid l ines) are located around the beg inning of Pipeline Road, SoberaniaNationa l Park, Panam a. Dotte d l ines indicate roads. Arrows indicate source ofreplacements (Ub indicates replacement by unbanded male of un know n terr i tory

status). Draw ing fro m Haverschmidt (1968).

W r e n , a b ou t 15% o f a du l t s swi t ch t e r ri to r ie s d u r ing the i r l if e tim e ,though som e pa i r s r e m a in toge the r on the sa m e t e r r i to r y f o r ove r f iveyears (S . Gi l l and B. J . M. S tu tch bu ry , un pu bl . ) .

H o w d o a d u l t s d e c i d e t o s w i t c h o r n o t t o s w i t c h t e r r it o r i e s? H o w d ot h e y a s s e s s t e r r i t o r y q u a l i t y o n n e i g h b o r i n g t e r r i t o r i e s ? D i r e c t e x p l o -r a t i o n o f t h e t e r r i t o r y is un l i ke l y , a n d m a y b e t o o r i s k y if p r e d a t i o n r i skis h i g h i n u n f a m i l i a r t e r r a in . D u s k y A n t b i r d s c a n n o t e a si ly l ea v e t h e i rc u r r e n t t e r r i t o r y t o e x p l o r e o t h e r s b e c a u s e t h e m a t e t h e y le a ve , e v e nf o r a m i n u t e o r t w o , w i ll b e g i n a d v e r t i s i n g f o r a n e w m a t e i f i ts s o n g s


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Figure 5.4Frequency distribution of time to be replaced for male (n = 9) and female (n = 5)

Dusky Antbirds experimen tally removed from territories.

or call notes are not answered (Morton and Derrickson 1996). Songoutput, especially during the dawn chorus, does not appear to be anaccurate, if indirect, measure of food abu nda nce on other territoriesbut the area for foraging was greater in those territories that birdsswitched to than it was on those territories they left (Morton e t a l .2000).

Mate aban don ment is an impor tant aspect of territory switching.Freed (1986) argued that permanent pair bonds had no intrinsicbenefit in House Wrens, but rather were forced on individuals by thelimited opportunities to switch territories. Switching territoriesusually means switching mates also, and the relative benefits to begained from each remain unknown. From the practical perspective, itwill be hard to tease apar t mat e choice from territo ry choice. In Buff-breasted Wrens some pairs have rema ined together, on the sameterritory, for over four years (S. Gill and B. J. M Stu tchbury, in prep.).Is this because they are, respectively, high quality mates or becausethey both occupy a high quality territory? Perhaps the solution is tomanipulate territory quality through food supplementation, to deter-mine whether one can induce territory switching. This assumes thatfood availability is a key feature of territory quality, and we do noteven know that that is true for tropical birds. Removal experimentshave revealed a wide variety of outcomes, from rapid mate/territory


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s w i t c h i n g in D u s k y A n t b i r d s a n d B u f f - b r e a s t e d W r e n s t o v e r y l it tl em a t e / t e r r i t o r y s w i t c h in g i n W h i t e - b e ll i e d A n t b i r d s. A c o m p a r a t i v ea p p r o a c h t o e x p l a in t h e s e d if f e r en c e s a m o n g s p e c ie s is a n o t h e r w a y t od e t e r m i n e h o w i m p o r t a n t m a t e a n d t e r r i t o r y q u a l i t y i s f o r t r o p i c a lb i rd s .

behavioral ecology of tropical birds - capitulo 5

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